1. Field of the Invention
The present invention relates to a ceramic electric-discharge lamp like a high pressure sodium lamp for example. In particular, the invention relates to a ceramic electric-discharge lamp which is composed a translucent ceramic tube and contains arc tube having at least two curved regions.
2. Description of the Related Art
Generally, any conventional high pressure sodium lamp has such a structure in which a pair of electrodes are airtightly sealed in both terminals of an arc lamp composed of a translucent alumina tube for example. Sodium as luminous material, mercury as buffer-gas material, and actuating rare gas, are respectively sealed in the arc tube. Since the translucent alumina tube is highly resistant against heat and corrosion from sodium, this tube is suited for composing an arc lamp of a conventional translucent aluminum lamp. Nevertheless, unlike conventional glass, translucent alumina tube softens itself in presence of high temperature. Availing of this physical property, conventionally, linear tubes produced by an extrusion molding process are widely used for composing envelopes. Consequently, arc tubes are generally provided with linear formation.
Recently, high pressure metallic vaporized electric discharge lamps are widely available for composing the light source of indoor illumination. Reflecting this demand, materialization of compact lamps is urged. On the other hand, applicability of high pressure sodium lamp to the indoor illumination has been studied among the concerned. To achieve this, contraction of the size of arc tube lamp is one of problems to solve.
Contraction of the length of the envelope makes up an idea for materializing compact arc lamps mentioned above. The shorter the length of the envelope, the shorter the distance between a pair of electrodes disposed near the both terminals of the arc lamp.
Nevertheless, it is necessary for any high pressure sodium lamps to suppress superficial temperature of the arc lamp below the predetermined degree. When contracting the distance between a pair of electrodes, since the tubular-wall load increases itself, the tubular diameter must be expanded. On the other hand, the wider the tubular diameter, the thicker the vaporized sodium layer, and as a result, luminous efficiency is sharply lowered by light-absorptive function of sodium.
If the tubular diameter were expanded in accordance with contracting the distance between electrodes, since the edge-corners of the arc tube are remote from the arc, temperature cannot rise. The corner portions bear the lowest temperature unlike any conventional system. This in turn causes amalgam of sodium to condense itself in the coldest region to interrupt evaporation. As a result, the arc tube cannot generate sufficient pressure for the vaporized sodium, thus eventually resulting in the failure to generate the predetermined lamp voltage.